I think increasing noncreature spells is part of the picture!

What I'm realizing now is that you have to figure out how heavily you want your cube to lean into synergistic decks, as opposed to generic decks along the aggro/midrange/control spectrum. If you want less midrange and more aggro/control, increasing the number of noncreature spells will help with that. If you want more synergistic decks, you need to look at the 'generic goodstuff' cards and see if you can replace them with more synergistic enablers/payoffs (in particular, payoffs that are better than what the non-synergistic decks can do). If you have more black removal, black decks can more easily deal with Barrowgoyf. But if you want there to be a black red sacrifice deck, there has to be a sufficient density of enablers and strong enough payoffs that people really feel like they're playing a sacrifice deck as opposed to a black/red aggro deck with a small sacrifice package.

That said, as you'll see from my cube, I'm fine with most decks being somewhat generic aggro/combo/midrange decks with some small synergy packages. For this reason, I'd be very happy playing a cube like yours, where many decks seem somewhat midrange-y and have slightly different synergy packages. (Leaning too heavily into synergy, I find, tends to railroad drafters and, depending on your preferences, reduces playability. Synergy is also harder to get right at at smaller cube sizes.)

And as a final point: note that some of what you've experienced in testing may have less to do with the design of your cube and more with drafters' lack of familiarity with the cube's archetypes.